window.addEvent('domready', function() {
	$('add').addEvent('click', function() {
		$('select1').getSelected().each(function(el) {
			el.inject($('select2'));
		});
	});
	$('remove').addEvent('click', function() {
		$('select2').getSelected().each(function(el) {
			el.inject($('select1'));
		});
	});
});



 <div class="holder">
	<select multiple="multiple" id="select1" name="select1[]">
		<option value="1">Option 1</option>
		<option value="2">Option 2</option>
		<option value="3">Option 3</option>
		<option value="4">Option 4</option>
	</select>
	<a href="javascript:;" id="add">add >></a>
</div>
<div class="holder">
	<select multiple="multiple" id="select2" name="select2[]">
		<option value="5">Option 5</option>
		<option value="6">Option 6</option>
	</select>
	<a href="javascript:;" id="remove"><< remove</a>
</div>

Multi-select transfer

javascript

JavaScript